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From: Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: Mon, 23 Mar 2026 12:29:50 +0100
Message-ID: 
 <CAKZiRmwuKVHaQa_3PirbBqzoCKciuqcn2Tn4Df4d8icq4e3DCA@mail.gmail.com>
Subject: Re: log XLogPrefetch stats at end of recovery
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>,
 Lakshmi N <lakshmin.jhs@gmail.com>,
	pgsql-hackers@postgresql.org
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On Sun, Mar 22, 2026 at 1:43=E2=80=AFAM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:

Hi,

> On Sat, Mar 21, 2026 at 1:16=E2=80=AFAM SATYANARAYANA NARLAPURAM
> <satyanarlapuram@gmail.com> wrote:
> >
> > > While investigating a long recovery, I noticed that XLogPrefetch stat=
s were not logged at the end of recovery. This log message will be useful t=
o understand how effective XLogPrefetch was during recovery. Adding a patch=
 to address this.
> >
> > Applied this patch and validated the log message. This log message appe=
ars to be useful to me, particularly while doing fleet wide analysis.
> >
> > 2026-03-20 23:33:13.756 PDT [2265441] LOG:  XLogPrefetcher stats: prefe=
tch=3D14, hit=3D6, skip_init=3D5, skip_new=3D28, skip_fpw=3D18, skip_rep=3D=
996
>
> This looks useful to understand how the prefetch helped during long recov=
eries.
>
> > I am wondering if we can periodically log this in standby mode as well,=
 not just before promoting?
>
> Timer-based startup progress messaging allows logging such things
> (ereport_startup_progress API). There was an attempt to enable "redo
> in progress" for standbys, but that seemed to flood the standby logs
> even at the default progress interval of 10 sec.
>
> Having said that, the prefetcher stats could be added to the existing
> ereport_startup_progress("redo in progress xxx") message that works
> for crash recoveries=E2=80=94however, I don't prefer doing a bunch of ato=
mic
> reads every progress interval of 10 sec.

> Therefore, logging at the end of recovery looks good to me.

+1 from me too to only of logging at the end of recovery (so -1 to logging
every now and then). If someone is interested in current state (or progress
over time) I think he can query pg_stat_recovery_prefetch view already, eve=
n
today, right?

> I reviewed the patch. I have the following comment:
>
> + elog(LOG, "XLogPrefetcher stats: prefetch=3D%lu, hit=3D%lu,
> skip_init=3D%lu, skip_new=3D%lu, skip_fpw=3D%lu, skip_rep=3D%lu",
>
> XLogPrefetcher is an internal data structure name, how about "redo
> prefetch stats: xxxx" to be consistent with other redo log messages?

+1

-J.